Plain English Meaning
A signal the aircraft sends out to ask the ground station 'how far am I?' The ground station hears the signal and sends one back, and the aircraft uses the round-trip time to work out the distance.
Full Definition
A coded radio pulse pair transmitted by an aircraft's DME (Distance Measuring Equipment) on a UHF frequency to a ground-based DME station, requesting a reply that the aircraft can use to calculate slant-range distance to the station.
Context Anchor
Seen in DME system descriptions, especially when explaining how the aircraft unit and the ground station work together to show distance.
Derivation
From the Latin 'interrogare,' meaning 'to question.' The aircraft is literally questioning the ground station -- 'how far am I from you?' -- and waiting for an answer.